Pollution control devices are employed on motor vehicles to control atmospheric pollution. Two types of devices are currently in widespread use--catalytic converters and diesel particulate filters or traps. Catalytic converters contain a catalyst, which is typically coated onto a monolithic structure mounted in the converter. The monolithic structures are typically ceramic, although metal monoliths have been used. The catalyst oxidizes carbon monoxide and hydrocarbons, and reduces the oxides of nitrogen in automobile exhaust gases to control atmospheric pollution. Diesel particulate filters or traps are wall flow filters which have honeycombed monolithic structures typically made from porous crystalline ceramic materials.
In the state of the art construction of these devices, each type of the these devices has a metal housing which holds within it a monolithic structure or element that can be metal or ceramic, and is most commonly ceramic. The ceramic monolith generally has very thin walls to provide a large amount of surface area so it is fragile and susceptible to breakage. It also has a coefficient of thermal expansion generally an order of magnitude less than the metal (usually stainless steel) housing in which it is contained. In order to avoid damage to the ceramic monolith from road shock and vibration, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monolith and the metal housing, ceramic mat or paste materials are typically disposed between the ceramic monolith and the metal housing. The process of placing or inserting the mounting material is also referred to as canning and includes such processes as injecting a paste into a gap between the monolith and the metal housing, or wrapping a sheet or mat material around the monolith and inserting the wrapped monolith into the housing.
Typically, the mounting materials include inorganic binders, inorganic fibers that may also serve as a binder, intumescent materials, and, optionally, organic binders, fillers, and other adjuvants. The materials are used as pastes, sheets, and mats. Ceramic mat materials, ceramic pastes, and intumescent sheet materials useful for mounting the monolith in the housing are described in, for example, U.S. Pat. Nos. 3,916,057 (Hatch et al.), 4,305,992 (Langer et al.), 4,385,135 (Langer et al.), 5,254,410 (Langer et al.), and 5,242,871 (Hashimoto et al.).
The use of synthetic mica and asbestos fibers as inorganic binders for inorganic fibrous mounting sheets is described in U.S. Pat. No. 3,001,571 (Hatch).
Paste compositions using micaceous materials are described in G.B. 1,522,646 (Wood).
U.S. Pat. Nos. 5,385,873 (MacNeil) and 5,207,989 (MacNeil) describe inorganic fibrous sheets bound by high aspect ratio vermiculite in paste for mounting monoliths in catalytic converters.
U.S. Patent No. 5,126,013 (Wiker et al.) describes the use of flocculating agents with chemically delaminated mica and/or vermiculite and fibers to make fire resistant papers which are generally thin in comparison to mats and sheets. The dual sequential flocculating system described helps to achieve rapid dewatering in making wet-laid materials and provide better sheet formation without clumping of the fibers.
U.S. Pat. No. 5,137,656 (Connor) describes the use of vermiculite lamellae having an internal sizing agent to make water resistant mineral products. Various means of producing the vermiculite lamellae are described.
One of the shortcomings of the state of the art pastes and mats used for mounting occurs because the exposed edges of the mounting materials are subject to erosion from the pulsating hot exhaust gases. Under severe conditions, over a period of time, the mounting materials can erode and portions of the materials can be blown out. In time, a sufficient amount of the mounting materials can be blown out and the mounting materials can fail to provide the needed protection to the monolith.
Solutions to the problem include the use of a stainless steel wire screen (see e.g., U.S. Pat. No. 5,008,086 (Merry)) and braided or rope-like ceramic (i.e., glass, crystalline ceramic, or glass-ceramic) fiber braiding or metal wire material (see, e.g., U.S. Pat. No. 4,156,533 (Close et al.)), and edge protectants formed from compositions having glass particles (see, e.g., EP 639701 A1 (Howorth et al.), EP 639702 A1 (Howorth et al.), and EP 639700 A1 (Stroom et al.)) to protect the edge of the intumescent mat from erosion by exhaust gases. These solutions employ the use of state of the art mounting materials as the primary support for the monolith.
The materials currently available as mounting mats and sheets typically contain inorganic binder material, refractory ceramic fibers in the range of about less than 5 micrometers in diameter, and other adjuvants. The inorganic binder materials and the refractory ceramic fibers provide the strength and resiliency needed for handling prior to canning as well as to keep the mounting material intact during the repeated heating and cooling cycles experienced in a catalytic converter. The fibers also bind the particulate materials to facilitate drainage of the compositions in making mounting mats in the state of the art wet-laid or papermaking process. The use of refractory ceramic fibers is undesirable and there remains an ongoing need for mounting materials, particularly for mats, having high performance without the use of small diameter, i.e., less than 5 micrometers, refractory ceramic fibers. Additionally, it is desirable for compositions used for mat mounting materials to have good drainage in the process of making the mats as well as resiliency for handling of the mats before mounting and in use in a catalytic converter.